Duplex system

ABSTRACT

A duplex system measuring and outputting at least one of a voltage and current is provided. A first measurement device measures at least one of the voltage and current. A second measurement device measures at least one of the voltage and current. A control device is disconnected from the first measurement device and performs a changeover to the second measurement device when the first measurement device abnormally operates. A filter filters a second measurement value of the second measurement device based on a first measurement value of the first measurement device to output a filtered value.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. §119(a), this application claims the benefit ofearlier filing date and right of priority to Korean Patent ApplicationNos. 10-2014-0030948 and 10-2014-0031310, filed on Mar. 17, 2014, thecontents of which are all hereby incorporated by reference herein intheir entirety.

BACKGROUND

The present disclosure relates to a duplex system, and moreparticularly, to a duplex system that includes a meter measuring acurrent and voltage.

A duplex system indicates providing and utilizing two components havingthe same function in order to enhance the reliability of a system. Theduplex system includes a spare device for each component so that theentire system does not need to stop when a component in a systemabnormally operates. When any one of components in a duplex system has atrouble, the duplex system immediately performs a changeover to a sparedevice to resume the operation of the duplex system. The duplex systemis costly but being widely utilized in a system that needs highreliability and cannot stop its operation.

Such a duplex system is also used in high voltage direct current (HVDC)transmission. The HVDC transmission is one of electricity transmissiontechniques. The HVDC transmission converts high-voltage alternatingcurrent power generated at a power station to highly-efficienthigh-voltage direct current power by using a power converter intotransmit the highly-efficient high-voltage direct current power.According to the HVDC transmission, the direct current power is thenre-converted into the alternating current power through the powerconverter in a desired region. The HVDC transmission is advantageous tolong-distance transmission because the amount of power loss is less incomparison to high voltage alternating current transmission.

An HVDC transmission device performing such HVDC transmission uses aninverter converting direct current power into alternating current powerand a converter converting the alternating current power into the directcurrent power. A meter measuring a current and voltage for the controlof the inverter and the converter measures the current and voltage at apoint of each device of a system and grid to which the HVDC transmissionis applied. The HVDC transmission needs high reliability. Thus, themeter measuring the current and voltage includes a duplex system. Whenthe meter normally operates and changes over, there is a risk that thereis a shock to the duplex system or the entire system normally operates.Thus, there is a need for a duplex system for preventing the shock orabnormal operation.

SUMMARY

Embodiments provide a duplex system that may minimize the shock of aduplex system when a changeover is performed due to the abnormaloperation of a system component. In particular, embodiments provide aduplex system that may minimize a shock to a duplex system when a metermeasuring a current and voltage normally operates and thus a changeoveris performed.

In one embodiment, a duplex system measuring and outputting at least oneof a voltage and current includes: a first measurement device measuringat least one of the voltage and current; a second measurement devicemeasuring at least one of the voltage and current; a control devicedisconnected from the first measurement device and performing achangeover to the second measurement device when the first measurementdevice abnormally operates; and a filter filtering a second measurementvalue of the second measurement device based on a first measurementvalue of the first measurement device to output a filtered value.

The control device establishes a connection to the filter before thecontrol device is disconnected from the first measurement device andperforms a changeover to the second measurement device, when the firstmeasurement device abnormally operates.

The control device may be disconnected from the filter, when a presettime after the connection to the filter is established elapses.

The filter may include at least one of a low pass filter, a linearfilter, and an interpolation filter.

The control device may determine that the first measurement deviceabnormally operates, when the first measurement value of the firstmeasurement device is a value having a difference to a previousmeasurement value of the first measurement device equal to or largerthan a preset reference value.

The duplex system may further include: a first buffer buffering thefirst measurement value of the first measurement device; and a secondbuffer buffering the second measurement value of the second measurementdevice, wherein the control device outputs a measurement value of thesecond measurement device based on the first measurement value bufferedand the second measurement device buffered.

The duplex system may further include an offset calculation unitcalculating a time offset being a difference in time between the firstmeasurement value buffered and the second measurement value buffered,wherein the control device outputs a measurement value of the secondmeasurement device based on the time offset.

The control device may correct the measurement value of the secondmeasurement device by the time offset to output a corrected earlymeasurement value, when the first measurement value buffered is earlierthan the second measurement value.

The control device may correct the measurement value of the secondmeasurement device by the time offset to output a corrected latemeasurement value, when the first measurement value buffered is laterthan the second measurement value.

In another embodiment, a method of operating a duplex system measuringand outputting at least one of a voltage and current includes: measuringat least one of the voltage and current by a first measurement deviceand a second measurement device; disconnecting from the firstmeasurement device and performing a changeover to the second measurementdevice when the first measurement device abnormally operates; andfiltering a second measurement value of the second measurement devicebased on a first measurement value of the first measurement device tooutput a filtered value.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features will be apparent fromthe description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a high voltage direct current (HVDC) transmission system towhich a duplex system according to an embodiment may be applied.

FIG. 2 is a flowchart of how a high voltage direct current (HVDC)transmission system to which a duplex system according to an embodimentmay be applied operates.

FIG. 3 is a block diagram of a measurement unit to which a duplex systemaccording to an embodiment is applied.

FIG. 4 is a flowchart of how a measurement unit to which a duplex systemaccording to an embodiment is applied operates.

FIG. 5 shows measurement values of a first measurement device and asecond measurement device that a measurement unit according to anembodiment includes.

FIG. 6 shows output values of a measurement unit when a control deviceof a duplex system according to an embodiment performs a changeover.

FIG. 7 is a block diagram of a measurement unit to which a duplex systemaccording to another embodiment is applied.

FIG. 8 is a flow chart of how a measurement unit to which a duplexsystem according to still another embodiment is applied operates.

FIG. 9 shows output values of a measurement unit when a duplex systemaccording to still another embodiment performs a changeover.

FIG. 10 shows measurement values of a first measurement device and asecond measurement device that a measurement unit according to anembodiment includes.

FIG. 11 shows output values of a measurement unit when a control deviceof a duplex system according to an embodiment performs a changeover.

FIG. 12 is a block diagram of a measurement unit to which a duplexsystem according to another embodiment is applied.

FIG. 13 is a flow chart of how a measurement unit to which a duplexsystem according to still another embodiment is applied operates.

FIG. 14 shows output values of a measurement unit when a duplex systemaccording to still another embodiment performs a changeover.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments are described below in detail with reference to theaccompanying drawings so that a person skilled in the art may easilypractice the embodiments. However, the present invention may beimplemented in many different forms and is not limited to embodimentsthat are described herein. In addition, parts irrelevant to descriptionsare not provided in the drawings in order to make the present inventionclear and similar parts throughout the disclosure have similar referencenumerals.

Also, when it is described that a part includes some elements, it shouldbe understood that it may not exclude but further include other elementsif there is no specific description contrary thereto.

FIG. 1 shows a high voltage direct current (HVDC) transmission system towhich a duplex system according to an embodiment may be applied.

The HVDC transmission system includes an alternating current powersource 110, a transmission-side transformer 120, a converter 130, aninverter 140, a reception-side transformer 150, a load 160, a controlunit 170, and a measurement unit 200.

The alternating current power source 110 generates alternating currentpower.

The transmission-side transformer 120 raises the voltage of generatedalternating current power. In this case, as the transmission-sidetransformer 120 raises the voltage, it is possible to enhance theefficiency of power transmission.

The converter 130 converts alternating current power into high voltageor extra-high voltage direct current power to transmit the extra-highvoltage direct current power. The converter 130 may include a thyristorvalve and rectifier. In this case, as the converter 130 convertsalternating current power into high voltage or extra-high voltage directcurrent power, it is possible to enhance the efficiency of powertransmission.

The inverter 140 converts direct current power into alternating currentpower.

The reception-side transformer 150 converts the voltage of thealternating current power into a rated voltage used for the load 160 tosupply alternating current obtained through conversion power to the load160.

The measurement unit 200 measures at least one of a voltage and currentin the HVDC transmission system 100. The measurement unit 200 in theembodiment according to FIG. 1 may measure converted alternating currentpower by the transmission-side transformer 120 without a limitation andthe measurement unit 200 may measure other parts in the HVDCtransmission system 100. The measurement unit 200 may be a duplex systemthat includes a plurality of measurement devices for the stability andreliability of a system. Related descriptions are provided in detailwith reference FIGS. 3 to 9.

The control unit 170 controls the operations of the converter 130 andthe inverter 140 based on a measurement value obtained by measuring atleast one of a voltage and current.

FIG. 2 is a flowchart of how a high voltage direct current (HVDC)transmission system to which a duplex system according to an embodimentmay be applied operates.

The alternating current power source 110 generates alternating currentpower in step S101.

The transmission-side transformer 120 raises the voltage of generatedalternating current power in step S103. The transmission-sidetransformer 120 may enhance the efficiency of power transmission byraising the voltage of alternating current power.

The measurement unit 200 measures at least one of a voltage and currentin the HVDC transmission system 100 to generate a measurement result instep S105.

The control unit 170 controls at least one of the converter 130 and theinverter 140 based on the measurement result in step S107.

The converter 130 converts alternating current power into extra-highvoltage or high voltage direct current power to transmit the extra-highvoltage or high voltage direct current power in step S109. It ispossible to enhance the efficiency of power transmission by convertingalternating current power into the extra-high voltage or high voltagedirect current power and transmitting the extra-high voltage or highvoltage direct current power.

The inverter 140 converts direct current power into alternating currentpower in step S111.

The reception-side transformer 150 converts the voltage of thealternating current power into a rated voltage used for the load 160 tosupply alternating current obtained through conversion power to the load160 in step S113.

FIG. 3 is a block diagram of a measurement unit to which a duplex systemaccording to an embodiment may be applied.

The measurement unit 200 includes a first measurement device 210, asecond measurement device 230, and a control device 250. In this case,the measurement device that the measurement unit 200 includes is notlimited to the first measurement device 210 and the second measurementdevice 230.

The first measurement device 210 and the second measurement device 230measure at least one of a voltage and current to generate a measurementvalue.

The control device 250 changes over to the second measurement device 230when the first measurement device 210 being currently used abnormallyoperates. In particular, the control device 250 may be disconnected fromthe first measurement device 210 being currently used, and connected tothe second measurement device 230 to generate a measurement value. Inthis case, the control device 250 may include a switch and the controldevice 250 may use the switch to be disconnected from the firstmeasurement device 210 and establish a connection to the secondmeasurement device 230. When the first measurement device 210 beingcurrently used measures a value having a difference to a previousmeasurement value equal to or larger than a predefined reference valueor stops measuring, the control device 250 may determine that the firstmeasurement device 210 abnormally operates. For example, when the firstmeasurement device 210 performs measurement and then stops measuring,the control device 250 may be disconnected from the first measurementdevice 210 and establish a connection to the second measurement device230 to generate a measurement value. The reason is to enhance thestability and reliability of the measurement unit as described earlier.Accordingly, the measurement unit 200 may normally operate even if anyone of a plurality of measurement devices abnormally operates.

FIG. 4 is a flowchart of how a measurement unit to which a duplex systemaccording to an embodiment is applied operates.

The first measurement device 210 and the second measurement device 230measure at least one of a voltage and current to generate a measurementvalue in step S201.

The control device 250 determines whether the first measurement device210 abnormally operates in step S203. In particular, when the firstmeasurement device 210 measures a value having a difference to aprevious measurement value equal to or larger than a predefinedreference value or stops measuring, the control device 250 may determinethat the first measurement device 210 abnormally operates.

When the first measurement device 210 abnormally operates, the controldevice 250 changes over to the second measurement device 230 in stepS205.

However, by a difference between measurement values of the firstmeasurement device 210 and the second measurement device 230 when achangeover is performed, the control unit 170 controlling the operationsof the converter 130 and the inverter 140 based on a measurement valueof the measurement unit 200 may abnormally operate. Related descriptionsare provided in detail with reference FIGS. 5 to 6.

FIG. 5 shows measurement values of a first measurement device and asecond measurement device that a measurement unit according to anembodiment includes.

FIG. 6 shows output values of a measurement unit when a duplex systemaccording to an embodiment performs a changeover.

Manufacturers of the control unit 170, the first measurement device 210,and the second measurement device 230 may be different from one another.Also, even if the first measurement device 210 and the secondmeasurement device 230 are products from the same manufacturer, thedevices may be different in model. In this case, even when the samepoint is measured at the same time, there may be a big differencebetween the output values of the first measurement device 210 and thesecond measurement device 230. Also, even when the first measurementdevice 210 and the second measurement device 230 are the same model fromthe same manufacturer, there may be a difference between output valuesbecause the first measurement device 210 and the second measurementdevice 230 operate independently. Curve A in FIG. 5 is formed by usingmeasurement values measured by the first measurement device 210 andcurve B in FIG. 5 is formed by using measurement values measured by thesecond measurement device 230. In the embodiment according to FIG. 5,curves A and B show a uniform difference in size. In this case, when achangeover is performed between the first measurement device 210 and thesecond measurement device 230, measurement values output by themeasurement unit 200 have a discontinuous point as shown in FIG. 6.Thus, when a changeover is performed between the first measurementdevice 210 and the second measurement device 230, a measurement valueoutput by the measurement unit 250 rapidly changes. When the measurementvalue rapidly changes, the control unit 170 controlling the operationsof the converter 130 and the inverter 140 based on the measurement valuemay abnormally operate. Therefore, there is a need for a duplex systemthat may prevent it.

FIG. 7 is a block diagram of a measurement unit to which a duplex systemaccording to another embodiment is applied.

The measurement unit 200 includes the first measurement device 210, thesecond measurement device 230, the control device 250, and a filter 270.The measurement unit 200 in FIG. 7 is the same as to the measurementunit 200 in the embodiment according to FIG. 3 in most components buthas a difference in that it includes the filter 270.

The control device 250 changes over to the second measurement device 230when the first measurement device 210 being currently used abnormallyoperates. In particular, the control device 250 may be disconnected fromthe first measurement device 210 being currently used, and connected tothe second measurement device 230 to generate a measurement value. Inthis case, the control device 250 establishes a connection to the filter270 before a changeover and inputs a measurement value of the firstmeasurement device 210 into the filter 270. The control device 250inputs a measurement value of the second measurement device 230 afterthe changeover and receives a filtered measurement value from the filter270. After the changeover, the control device 250 may be disconnectedfrom the filter 270 after a preset reference time elapses.

The filter 270 filters a measurement value of the second measurementdevice 230 based on a measurement value of the first measurement device210 when a changeover is performed to decrease a difference between ameasurement value before the changeover and a measurement value afterthe changeover that are output by the control device 250. In particular,the filter 270 filters a measurement value of the second measurementdevice 230 based on a measurement value of the first measurement device210 to decrease a difference between a measurement value before thechangeover and a measurement value after the changeover that are outputby the control device 250. In particular, the filter 270 may include atleast one of a low pass filter, a linear filter, and an interpolationfilter. Accordingly, the filter 270 prevents the discontinuity ofmeasurement values when a changeover is performed. Thus, it is possibleto prevent the malfunction of the control unit 170 resulting from arapid change in a measurement value.

There may be a case where there is an difference in time as well as andifference in size between a measurement value of the first measurementdevice 210 and a measurement value of the second measurement device 230.In order to prevent it, the measurement unit 200 may include a firstbuffer (not shown), a second buffer (not shown), and an offsetmeasurement unit (not shown). The first buffer (not shown) buffers ameasurement value of the first measurement device 210. In particular,the first buffer (not shown) stores a measurement value of the firstmeasurement device 210 for a certain time. The second buffer (not shown)buffers a measurement value of the second measurement device 230. Inparticular, the second buffer (not shown) stores a measurement value ofthe second measurement device 230 for a certain time. An offsetcalculation unit (not shown) calculates a difference in time between avalue buffered in the first buffer (not shown) and a value buffered inthe second buffer (not shown) to output an offset that is a differencein time between a measurement value of the first measurement device 210and a measurement value of the second measurement device 230. Thecontrol device 250 may output a measurement value of the secondmeasurement device 230 after a changeover based on a bufferedmeasurement value of the first measurement device and a bufferedmeasurement value of the second measurement device. In particular, thecontrol device 250 may output a measurement value of the secondmeasurement device 230 after the changeover based on an time offsetreceived from the offset calculation unit (not shown). In particular,the control device 250 may correct the measurement value of the secondmeasurement device 230 after the changeover by the time offset receivedfrom the offset calculation unit (not shown). When a measurement valueof the first measurement device 210 is earlier than a measurement valueof the second measurement device 230, the control device 250 may movethe measurement value of the second measurement device 230 as early asthe time offset. When a measurement value of the first measurementdevice 210 is later than a measurement value of the second measurementdevice 230, the control device 250 may move the measurement value of thesecond measurement device 230 as late as the time offset. Accordingly,the filter 270 prevents the discontinuity of measurement values when achangeover is performed. Thus, it is possible to prevent the malfunctionof the control unit 170 resulting from the discontinuity of measurementvalues.

FIG. 8 is a flow chart of how a measurement unit to which a duplexsystem according to still another embodiment is applied operates.

The first measurement device 210 and the second measurement device 230measure at least one of a voltage and current to generate a measurementvalue in step S301.

The control device 250 determines whether the first measurement device210 abnormally operates in step S303. In particular, when the firstmeasurement device 210 measures a value having a difference to aprevious measurement value equal to or larger than a predefinedreference value or stops measuring, the control device 250 may determinethat the first measurement device 210 abnormally operates.

When the first measurement device 210 abnormally operates, the controldevice 250 establishes a connection to the filter 270 in step S305. Asthe connection to the filter 270 is established, the control device 250inputs a measurement value of the first measurement device 210 into thefilter.

The control device 250 performs a changeover to the second measurementdevice 230 in step S307. As the changeover to the second measurementdevice 230 is performed, the control device 250 inputs a measurementvalue of the second measurement device 250 into the filter.

The filter 270 filters a measurement value of the second measurementdevice 230 based on a measurement value of the first measurement device210 when a changeover is performed to decrease a difference between ameasurement value before the changeover and a measurement value afterthe changeover that are output by the control device 250 in step S309.Te filter 270 may include at least one of a low pass filter, a linearfilter, and an interpolation filter. Thus, the filter 270 may perform atleast one of low-band filtering, linear filtering and interpolationfiltering. Accordingly, the filter 270 prevents the discontinuity ofmeasurement values when a changeover is performed. Thus, the filter 270may prevent the malfunction of the control unit 170 resulting from arapid change in a measurement value.

The control device 250 is disconnected from the filter 270 in step S311.After a connection is generated and then a preset reference timeelapses, the control device 250 may be disconnected from the filter 270.As the connection to the filter 270 is disconnected, the control device250 outputs a measurement value of the first measurement device 230 bynot using the filter. The reason is to prevent deformation appearingwhen the measurement value of the second measurement device 230 uses thefilter. The reason is also to prevent an output of a measurement valueby the control device 250 from becoming delayed due to the filter.

FIG. 9 shows output values of a measurement unit when a duplex systemaccording to still another embodiment performs a changeover.

The output values of the measurement unit 250 as shown in FIG. 9 do notexperience discontinuity between before and after the changeover unlikethose in FIG. 6. As described earlier, the reason is because the filter270 filters a measurement value of the second measurement device 230based on a measurement value of the first measurement device 210 when achangeover is performed to decrease a difference between a measurementvalue before the changeover and a measurement value after the changeoverthat are output by the control device 250. Thus, it is possible toprevent the output values of the measurement unit 200 from experiencingdiscontinuity.

FIG. 10 shows measurement values of a first measurement device and asecond measurement device that a measurement unit according to anembodiment includes.

FIG. 11 shows output values of a measurement unit when a duplex systemaccording to an embodiment performs a changeover.

Since the first measurement device 210 and the second measurement device230 operate independently when the same point is measured at the sametime, the output values of the first measurement device 210 and thesecond measurement device 230 may have a difference in time according tothe hardware characteristic of a device. Also, even if the firstmeasurement device 210 and the second measurement device 230 areproducts from the same manufacturer, the devices may be different inproduct model. Thus, the measurement values of the first measurementdevice 210 and the second measurement device may have a difference intime. In this case, there is a gap between the measurement values. Inparticular, when the control device 250 is disconnected from the firstmeasurement device 210 and establishes a connection to the secondmeasurement device 230, there is a gap between measurement values outputby the measurement unit 200. Curve A in FIG. 10 is formed by usingmeasurement values measured by the first measurement device 210 andcurve B in FIG. 10 is formed by using measurement values measured by thesecond measurement device 230. In the embodiment according to FIG. 10,the first measurement device 210 and the second measurement device 230output the same measurement value at a certain time interval. In theembodiment according to FIG. 10, the first measurement device 210generates a measurement value a bit earlier than the second measurementdevice 230. Thus, when a changeover is performed between the firstmeasurement device 210 and the second measurement device 230,measurement values output by the measurement unit 200 may have a gap asshown in FIG. 6. Thus, the control unit 170 controlling the operationsof the converter 130 and the inverter 140 based on the measurement valuemay abnormally operate. Therefore, there is a need for a duplex systemthat may prevent it.

FIG. 12 is a block diagram of a measurement unit to which a duplexsystem according to another embodiment is applied.

A measurement unit 300 includes the first measurement device 310, thesecond measurement device 330, the control device 350, a first buffer370, a second buffer 380 and an offset calculation unit 390. Themeasurement unit 300 in FIG. 12 is the same as the measurement unit 200in the embodiment according to FIG. 3 in most components but has adifference in that it includes the first buffer 370, the second buffer380, and the offset calculation unit 390.

The first buffer 370 buffers a measurement value of the firstmeasurement device 310. In particular, the first buffer 370 stores ameasurement value of the first measurement device 310 for a certaintime.

The second buffer 380 buffers a measurement value of the secondmeasurement device 330. In particular, the second buffer (380) stores ameasurement value of the second measurement device 330 for a certaintime.

The offset calculation unit 390 calculates a difference in time betweena value buffered in the first buffer 370 and a value buffered in thesecond buffer 380 to output an offset that is a difference in timebetween a measurement value of the first measurement device 310 and ameasurement value of the second measurement device 330.

The control device 350 changes over to the second measurement device 330when the first measurement device 310 being currently used abnormallyoperates. In particular, the control device 350 may be disconnected fromthe first measurement device 310 being currently used, and connected tothe second measurement device 330 to generate a measurement value. Thecontrol device 350 may output a measurement value of the secondmeasurement device 330 after a changeover based on a bufferedmeasurement value of the first measurement device and a bufferedmeasurement value of the second measurement device. In particular, thecontrol device 350 may output a measurement value of the secondmeasurement device 330 after the changeover based on a time offsetreceived from the offset calculation unit 390. In particular, thecontrol device 350 may correct the measurement value of the secondmeasurement device 330 after the changeover by the time offset receivedfrom the offset calculation unit 390. When a measurement value of thefirst measurement device 310 is earlier than a measurement value of thesecond measurement device 330, the control device 350 may move themeasurement value of the second measurement device 330 as early as thetime offset. When a measurement value of the first measurement device310 is later than a measurement value of the second measurement device330, the control device 350 may move the measurement value of the secondmeasurement device 330 as late as the time offset. Accordingly, thecontrol device 350 prevents the gap between measurement values when achangeover is performed. Thus, it is possible to prevent the malfunctionof the control unit 170 resulting from the gap between measurementvalues.

Also, there may be a case where there is an difference in size betweenmeasurement values as well as an difference in time as describedearlier. When there is the difference in size between measurementvalues, there may be the discontinuity of measurement values when achangeover is performed. In order to prevent it, the measurement unit300 may further include a filter (not shown). The filter (not shown)filters a measurement value of the second measurement device 330 basedon a measurement value of the first measurement device 310 when achangeover is performed to decrease a difference between a measurementvalue before the changeover and a measurement value after the changeoverthat are output by the control device 350. The filter (not shown) mayinclude at least one of a low pass filter, a linear filter, and aninterpolation filter. Thus, the filter (not shown) may perform at leastone of low-band filtering, linear filtering and interpolation filtering.Accordingly, the filter (not shown) prevents the discontinuity ofmeasurement values when a changeover is performed. Thus, the filter 270may prevent the malfunction of the control unit 170 resulting from arapid change in a measurement value. In this case, the control device350 may establish a connection to the filter (not shown) before achangeover and input a measurement value of the first measurement device310 into the filter (not shown). The control device 350 inputs ameasurement value of the second measurement device 330 after thechangeover and receives a filtered measurement value from the filter(not shown). After the changeover, the control device 350 may bedisconnected from the filter (not shown) after a preset reference timeelapses. Thus, the measurement unit 300 to be described in FIGS. 12 to14 may include all components of the measurement unit 200 as describedin FIGS. 7 to 9 and equally perform the operations of the measurementunit 200.

FIG. 13 is a flow chart of how a measurement unit to which a duplexsystem according to still another embodiment is applied operates.

The first measurement device 310 and the second measurement device 330measure at least one of a voltage and current to generate a measurementvalue in step S501.

The first buffer 370 buffers a measurement value of the firstmeasurement device 310 and the second buffer 380 buffers a measurementvalue of the second measurement device 330 in step S503. In particular,the first buffer 370 may store a measurement value of the firstmeasurement device 310 for a certain time and the second buffer 380 maystore a measurement value of the second measurement device 330 for acertain time.

The offset calculation unit 390 calculates a time offset that is adifference in time between a buffered measurement value of the firstmeasurement device 310 and a measurement value of the second measurementdevice in step S505.

The control device 350 determines whether the first measurement device310 abnormally operates in step S507. In particular, when the firstmeasurement device 310 measures a value having a difference to aprevious measurement value equal to or larger than a predefinedreference value or stops measuring, the control device 350 may determinethat the first measurement device 310 abnormally operates.

When the first measurement device 310 abnormally operates, the controldevice 350 changes over to the second measurement device 330 in stepS509. In particular, when the first measurement device 310 abnormallyoperates, the control unit 350 may perform a changeover to the secondmeasurement device 330. In particular, the control device 350 may bedisconnected from the first measurement device 310 being currently used,and connected to the second measurement device 330 to generate ameasurement value.

The control device 350 outputs a measurement value of the secondmeasurement device 330 based on the time offset in step S511. Inparticular, the control device 350 may correct the measurement value ofthe second measurement device 330 after the changeover by the timeoffset received from the offset calculation unit 390. When a measurementvalue of the first measurement device 310 is earlier than a measurementvalue of the second measurement device 330, the control device 350 maymove the measurement value of the second measurement device 330 as earlyas the time offset. When a measurement value of the first measurementdevice 310 is later than a measurement value of the second measurementdevice 330, the control device 350 may move the measurement value of thesecond measurement device 330 as late as the time offset. Accordingly,the filter 270 prevents the discontinuity of measurement values when achangeover is performed. Thus, it is possible to prevent the malfunctionof the control unit 170 resulting from a rapid change in a measurementvalue.

FIG. 14 shows output values of a measurement unit when a duplex systemaccording to still another embodiment performs a changeover.

The output values of the measurement unit 250 as shown in FIG. 14 do notexperience a gap between before and after the changeover unlike those inFIG. 11. As described earlier, the control device 350 outputs themeasurement value of the second measurement device 330 based on a timeoffset related to a buffered measurement value of the first measurementdevice 310 and a measurement value of the second measurement device 330when a changeover is performed. Thus, a difference in time between ameasurement value before the changeover and a measurement value afterthe changeover that are output by the measurement unit 300 may becorrected. Thus, it is possible to prevent the output values of themeasurement unit 300 from experiencing a gap.

The characteristics, structures, and effects described in theembodiments above are included in at least one embodiment but are notlimited to one embodiment. Furthermore, the characteristic, structure,and effect illustrated in each embodiment may be combined or modifiedfor other embodiments by a person skilled in the art. Thus, it would beconstrued that contents related to such a combination and such avariation are included in the scope of embodiments.

Embodiments are mostly described above. However, they are only examplesand do not limit the present invention. A person skilled in the art mayappreciate that many variations and applications not presented above maybe implemented without departing from the essential characteristic ofembodiments. For example, each component particularly represented inembodiments may be varied. In addition, it should be construed thatdifferences related to such a variation and such an application areincluded in the scope of the present invention defined in the followingclaims.

A duplex system according to an embodiment provides a duplex system thatmay minimize a shock to the duplex system when a changeover is performeddue to an abnormal operation of a system component. In particular, it ispossible to provide a duplex system that may minimize a shock to aduplex system due to a changeover when a measurement device measuring acurrent and voltage normally operates and thus there is the changeover.

Also, although embodiments which are applied to the HVDC transmissionsystem are described in order to particularly describe the duplex systemof the present invention, the present invention is not limited theretoand the duplex system of the present invention may be used for othersystems.

What is claimed is:
 1. A duplex system measuring and outputting at leastvoltage or current, the duplex system comprising: a first measurementdevice measuring at least the voltage or current to generate a firstmeasurement value; a second measurement device measuring at least thevoltage or current to generate a second measurement value; a controldevice outputting the first measurement value or the second measurementvalue, the control device disconnected from the first measurement deviceand performing a changeover from the first measurement device to thesecond measurement device when a difference between a present firstmeasurement value and a previous first measurement value is equal to orgreater than a predefined reference value or the first measurementdevice stops measuring voltage or current values; and a filter filteringthe second measurement value based on the first measurement value todecrease a difference between the first measurement value or secondmeasurement value output by the control device before the changeover andthe first measurement value or second measurement value output by thecontrol device after the changeover.
 2. A duplex system measuring andoutputting at least voltage or current, the duplex system comprising: afirst measurement device measuring at least the voltage or current togenerate a first measurement value; a second measurement devicemeasuring at least the voltage or current to generate a secondmeasurement value; a control device disconnected from the firstmeasurement device and performing a changeover to the second measurementdevice when the first measurement device abnormally operates; and afilter filtering the second measurement value based on the firstmeasurement value, wherein the control device establishes a connectionto the filter before the control device is disconnected from the firstmeasurement device and the changeover to the second measurement deviceis performed.
 3. The duplex system according to claim 2, wherein thecontrol device is disconnected from the filter a preset time after theconnection to the filter is established.
 4. The duplex system accordingto claim 2, wherein the filter comprises at least a low pass filter, alinear filter or an interpolation filter.
 5. The duplex system accordingto claim 2, wherein the control device determines that the firstmeasurement device abnormally operates when a difference between apresent first measurement value and a previous first measurement valueis equal to or greater than a preset reference value.
 6. The duplexsystem according to claim 2, further comprising: a first bufferbuffering the first measurement value to generate a first bufferedvalue; and a second buffer buffering the second measurement value togenerate a second buffered value, wherein the control device outputs thesecond measurement value based on the first buffered value and thesecond buffered value.
 7. The duplex system according to claim 6,further comprising an offset calculation unit calculating a time offsetthat is a difference in time between the first buffered value bufferedand the second buffered value, wherein the control device outputs thesecond measurement value based on the calculated time offset.
 8. Theduplex system according to claim 7, wherein the control device correctsthe second measurement value according to the time offset to output acorrected early measurement value when the first buffered value isearlier than the second measurement value.
 9. The duplex systemaccording to claim 7, wherein the control device corrects the secondmeasurement value according to the time offset to output a correctedlate measurement value when the first buffered value is later than thesecond measurement value.
 10. A method of operating a duplex systemmeasuring and outputting at least voltage or current, the methodcomprising: measuring at least the voltage or current by a firstmeasurement device to generate a first measurement value; measuring atleast the voltage or current by a second measurement device to generatea second measurement value; outputting the first measurement value orthe second measurement value; disconnecting from the first measurementdevice and performing a changeover from the first measurement device tothe second measurement device when a difference between a present firstmeasurement value and a previous first measurement value is equal to orgreater than a predefined reference value or the first measurementdevice stops measuring voltage or current values; and filtering thesecond measurement value based on the first measurement value todecrease a difference between the first measurement value or secondmeasurement value output before the changeover and the first measurementvalue or second measurement value output after the changeover.
 11. Amethod of operating a duplex system measuring and outputting at leastvoltage or current, the duplex method comprising: measuring at leastvoltage or current by a first measurement device to generate a firstmeasurement value; measuring at least the voltage or current by a secondmeasurement device to generate a second measurement value; establishinga connection to a filter when the first measurement device abnormallyoperates; filtering the second measurement value of based on the firstmeasurement value; disconnecting from the first measurement device andperforming a changeover to the second measurement device.
 12. The methodaccording to claim 11, further comprising disconnecting from the filtera preset time after the connection to the filter is established.
 13. Themethod according to claim 11, wherein the filter comprises at least alow pass filter, a linear filter or an interpolation filter.
 14. Themethod according to claim 11, wherein disconnecting from the firstmeasurement device and performing the changeover to the secondmeasurement device comprises determining that the first measurementdevice abnormally operates when a difference between a present firstmeasurement value and a previous first measurement value is equal to orgreater than a preset reference value.
 15. The method according to claim11, further comprising: buffering the first measurement value togenerate a first buffered value; and buffering the second measurementvalue to generate a second buffered value, wherein filtering the secondmeasurement value comprises outputting the second measurement valuebased on the first buffered value and the second buffered value.
 16. Themethod according to claim 15, wherein outputting the second measurementvalue based on the first buffered value and the second buffered valuecomprises: calculating a time offset that is a difference in timebetween the first buffered value and the second buffered value; andoutputting the second measurement value based on the calculated timeoffset.
 17. The method according to claim 16, wherein outputting thesecond measurement value based on the calculated time offset comprisescorrecting the second measurement value according to the time offset tooutput a corrected early measurement value when the first buffered valueis earlier than the second measurement value.
 18. The method accordingto claim 16, wherein outputting the second measurement value based onthe first buffered value and the second buffered value comprisescorrecting the second measurement value according to the time offset tooutput a corrected late measurement value when the first buffered valueis later than the second measurement value.